Generally, pulse radar apparatuses transmit pulsed high-frequency signals from antennas and receive reflection signals from target objects in the surroundings.
Generally, a radar apparatus includes a display device for displaying a signal received by an antenna as a radar image. A user of the radar apparatus can check the displayed radar image to confirm a target object in the surroundings. There exist various modes of displaying the signal received by the antenna, for example, JP1996-015419A and JP1993-012640B disclose an A-scope display and a PPI (Plan Position Indicator) scope display.
In the A-scope display, by taking a distance to a target object in the horizontal axis and a signal level in the vertical axis, the reception signal is displayed in a chart. The A-scope display has a merit of easily distinguishing a target object echo from a direction in which the antenna is oriented, and it is used in a tracking radar (JP1993-012640B). However, since antennas of radar apparatuses which are generally used in ships and the like always rotate, even if the signal received by the antenna is displayed in the A-scope in real time, the A-scope display is hardly of any use in a target detection. Therefore, the A-scope display is hardly used in ship radars (see L.10, 7th column of JP1995-027020B).
In the PPI-scope display, the signal received by the antenna is displayed by being plotted in a two-dimensional rectangular coordinate system. The PPI-scope display has a merit of allowing a user to easily instinctively confirm the position of the target object in a horizontal plane. Therefore, ship radars generally use the PPI-scope display.
Meanwhile, the level of a reflection signal from a target object in a close distance is high, and the level of a reflection signal from a target object in a far distance is weak. Therefore, if a radar image in the PPI-scope is generated by using the received signal as it is, since the view of the echo image becomes completely different depending on whether the target object is in a close distance or a far distance, the radar image will be difficult to be viewed.
Therefore, in the field of radar apparatus, processing which is a so-called STC (Sensitivity Time Control) of adjusting the level of the reception signal according to the distance to the target object is generally performed. Specifically, the sensitivity is decreased when the signal from a close distance is received and the sensitivity is increased as the distance is farther. For example, JP1997-072958A discloses such an STC.
With the radar apparatus, an adjustment of a characteristic of the STC becomes important to facilitate viewing of the PPI-scope radar image on display. In this regard, JP1997-072958A discloses a configuration in which the STC characteristic can be changed by adjusting it while looking at a display screen of a CRT (the display screen with the PPI-scope radar image displayed thereon) so that the image which can be easily viewed can be obtained.
With the radar apparatus, by the STC adjustment, if the echo image of the target object is eliminated along with clutter, there may be a case where an important target object cannot be confirmed with the radar image. Therefore, when adjusting the STC, the adjustment to appropriately remain the echo image of the required target object is requested.
However, with the configuration of JP1997-072958A, no method can confirm that only clutter is eliminated (the echo image of the target object is appropriately remained) by the STC processing. Therefore, whether the adjustment of the STC is appropriately performed cannot be evaluated instantly.
All told, even with the configuration which enable to adjust the STC while looking at the radar image displayed on the CRT as JP1997-072958A, in order to display the radar image so that only clutter is eliminated and the echo image of the target object is remained, the adjustment of the STC requires trial-and-error. Thus, with the configuration of JP1997-072958A, the adjustment of the STC is considered no easy.